Atomized polysulfide used ethylene steam cracker

ABSTRACT

Dialkyl polysulfides can be added to hydrocarbon gas fed streams for alkene steam crackers for the petrochemical industry. The polysulfides act as sulfiding agents for surfaces of the steam crackers and thereby reduce coking and CO generation. A preferred polysulfide is a di-t-butyl polysulfide. Such compounds tend to have lower vapor pressure, reasonable viscosities, and are relatively nonflammable as compared to the lower molecular weight sulfur containing compounds.

FIELD OF INVENTION

[0001] Small amounts of sulfur containing chemicals are used asadditives in ethylene steam crackers to prevent coking and otherundesired chemical reactions. The sulfur sources are typicallycharacterized as, sulfiding agents. Low molecular weight sulfurcontaining agents present fire and toxicological hazards due to theirvolatility. The polysulfidic sulfur containing agents have lowervolatility and lower flammability.

BACKGROUND OF THE INVENTION

[0002] The use of sulfur compounds in feed streams to ethylenesteamcrackers (both thermal and catalytic) has been known for many yearsto reduce coking rates and to reduce CO (carbon monoxide) production inthe pyrolysis section of steamcracker furnaces. Others characterize thecontribution of the sulfur compounds with improved reaction speed andproduct selectivity along with delaying or decreasing the frequency ofde-coking of the reactors (lengthening the time between de-cokingprocedures). The sulfur sources are called sulfiding agents and arebelieved to decompose to form sulfidic surfaces on the insides of thereactors.

[0003] Sources of sulfur compounds have included 1) naturally occurringsulfur in feed (i.e. use of sour feed), 2) inexpensive odiferous sulfurcompounds, which are also used for hydrotreater catalyst sulfiding. Inparticular the natural sulfur in good feedstocks has worked well withCo/Mo catalysts. As poorer quality feedstocks were used the sulfurcontent had higher thermal stability and required extremely hightemperatures to activate the catalyst. Also as more sophisticatedcatalysts were made with greater hydrogenating and cracking power, theybenefited from higher degrees of sulfurization than was possible withthe feedstock. The preferred inexpensive odiferous sulfur compoundsinclude DMDS (dimethyl disulfide) or DMS (dimethyl sulfide).

[0004] DMDS has a flash point of 15° C., a boiling point of 110° C., astrong odor and is a powerful solvent for many polymers. DMS has a flashpoint of <−18° C. and a boiling point of 36-39° C. Both compounds havestrong odors, require grounded equipment, and require operators incontact or potential contact with the chemicals to have goggles, gloves,and an air supplied respirator.

SUMMARY OF THE INVENTION

[0005] It has been discovered that low viscosity relatively nonvolatilepolysulfides can be used as sulfiding agents in gas fed alkene(ethylene) thermal steam crackers. Preferable embodiments arenon-catalytic ethylene steam crackers which rely predominantly onthermal cracking, although embodiments using catalysts are notprecluded. While the polysulfides, due to their molecular weight,typically can't be volatilized in the 20-40° C. hydrocarbon gas streamsfeeding the gas fed ethylene crackers, they can be dispersed as a finespray using a spray nozzle or atomizer. These fine sprays can be carriedinto the heated portion of the reactor where they readily volatilizedand thermally decomposed to sulfide the inner surfaces of the reactor.These reactors typically are not catalytic (using a catalyst) but areusually simple thermal crackers.

DETAILED DESCRIPTION OF THE INVENTION

[0006] The use of sulfur compounds in thermal and catalytic crackers iswell known. The sulfur compounds are thermally decomposed and act assulfiding agents. In the past there have been conflicting objectiveswith these sulfur compounds. It was desirable to use volatile sulfurcompounds with low thermal decomposition temperatures such as dimethyldisulfide. While such compounds that were easily volatilized andthermally decomposed to achieve the objective, they also had low flashpoints, extremely objectionable odor, and toxicity (the toxicity wasespecially troublesome as these compounds have high vapor pressures).

[0007] Among the low molecular weight organic compounds with relativelyhigh amounts of the active sulfur components one can identifydimethyldisulfide, dimethylsulfide, hydrogen sulfide, carbon disulfide,methylmercaptan, ethylmercaptan, and n-butylmercaptan. Dimethyldisulfideis preferred among these compounds for a variety of reasons, includingits low thermal decomposition temperature, relative to the others, andthe fact that it produces very little unsaturated hydrocarbon compoundson thermal decomposition. These unsaturated hydrocarbon compounds arethought to be significant contributors to coking in crackers, which isundesirable. All of these low molecular weight compounds havesignificant vapor pressure at 20° C. and flash points of 16° C. or lessmaking them hazardous to ship and handle.

[0008] In recent years some ethylene producers have considered usinglower odor polysulfides in liquid fed (naphtha or gas oil) ethylenesteam crackers to minimize odor and decrease worker exposure to thevolatile and more hazardous low molecular weight sulfur compounds. Thepolysulfides, such as t-nonyl polysulfide, were soluble in the liquidsand could be conveniently added to the liquid feed prior to addition tothe steam cracker. Since the polysulfides are liquids at 20-25° C. andhave fairly low vapor pressure at those temperatures, there was no easyway to add polysulfides to gas feeds for ethylene crackers. Thepolysulfides, since they typically carry 2 to 8 sulfur atoms permolecule, have comparable sulfur content to dimethylsulfide (DMScontains 51% S, within the polysulfide range, while DMDS contains 68%S).

[0009] It has been discovered that some of the less viscouspolysulfides, such as SulfrZol 54 di-t-butyl polysulfide, having aviscosity of 14 cps at 20° C. can be finely dispersed or atomized intothe gas feed streams for an alkene steam cracker such as an ethylenesteam cracker. Desirable polysulfides for this purpose have viscositiesbetween 1 and 50 cps at 20° C. as measured by a Brookfield viscometer.This can be accomplished by inserting a metering system on thepolysulfide to add the correct amount of polysulfide and a nozzle oratomizer, drawing from the polysulfide source and inserted in thehydrocarbon gas stream somewhere before the alkene steam cracker.Desirably the nozzle or atomizer is inserted a few inches or a few feet(about 1 to about 50 or 100 feet) before the manifold that immediatelyprecedes the various reactor tubes of the alkene steam cracker. Theproximity of the nozzle or atomizer to the manifold can minimize thetime that the fine dispersion of polysulfide needs to stay suspended inthe gas stream before the polysulfide enters the reactor tubes where itis almost simultaneously volatilized and decomposed. If the nozzle oratomizer is moved further upstream from the manifold the quality of thedispersion of polysulfide would advisably be increased to minimize theamount of polysulfide that is deposited on the walls of the pipe used tofeed the manifold.

[0010] The polysulfides of interest would typically have the formulaR—S_(x)—R where R is a linear or branched alkyl of 3 to 15 carbon atomsand x is either an integer between 3 and 8 or R—S_(x)—R is a blend ofcompounds where x varies between 1 and 8. In a typical SulfrZol 54 typeof product x can be 4 for about 30-50 number percent of the moleculesand values of 3 to 6 for about 80-95 number percent of the molecules.Trace amounts of molecules where x is 1, 2, 7, or 8 are present.Desirably R would be a linear or branched alkyl of 3 to 10 carbon atoms,and most preferably t-butyl. In a preferred embodiment at least 50% ofthe R groups are t-butyl groups.

[0011] The polysulfide may be formed into a dispersion by a variety oftechniques. If the polysulfide is of sufficiently low viscosity it mightsimply be sprayed as a fine mist. A gas assisted spray nozzle oratomizer may be used to provide further shear to the liquid polysulfideto reduce the particle size. The gas of the gas assist might includerelatively inert gases such as nitrogen or argon or hydrocarbon gasessuch as natural gas or propane. The hydrocarbon gas for gas assistedspray nozzles desirably has an average carbon length of 4 or less. Thehydrocarbon gas feed stream typically operates from a few poundspressure up to several hundred pounds pressure with a preferred rangebeing from about 50 psig to about 150 psig. The hydrocarbon gas feedstreams typically are fed at a temperature of less than 100° C. and moretypically at ambient outside temperature (e.g. 10-40° C.). Multiplespray nozzles or atomizer tips may be used to increase the efficiency offorming the dispersion. A diluent may be added to the polysulfide tofurther reducing the viscosity of the polysulfide. A supercriticalliquid may be added to the polysulfide to both reduce the viscosity andto volatilize from any droplets formed, further reducing their size.

[0012] It is anticipated that the dispersion of polysulfide in the gasstream would exist for about 1 to about 30 seconds, more preferably fromabout 5 to about 20 seconds before the polysulfide reached the warmerzones of the reactor tubes where it would be volatilized and thermallydecomposed. The spray droplet sizes would be determined based upon thistime and the calculated flow rate in the hydrocarbon gas stream to thealkene steam cracker.

[0013] The amount of polysulfidic compound added to the hydrocarbon feedto the alkene steam cracker would depend on the amount of sulfur alreadyexisting in the hydrocarbon feed and the sulfur requirements of theparticular alkene steam cracker. More sulfur, in the form of apolysulfidic compound, may be added initially to a cracker after cokecleaning or other assembly or maintenance operations. In this embodimenttypically at least 10 ppm or 50 ppm of sulfur is added in the form of apolysulfidic compound, based on the weight of the hydrocarbon gasstream. More desirably from about 20 to about 300 ppm of sulfur is addedas the polysulfidic compound and preferably from about 50 to about 250ppm of sulfur is added as the polysulfidic compound. Typically theamount of sulfur for continuous production is adjusted between about 50and 400 ppm based on the weight of the hydrocarbon feed stream, and moredesirably from about 75 to about 300 ppm. These amounts are adjustedbased on the perceived rate of coking occurring in the reactor and theamount of CO (carbon monoxide) being generated by the process. Addingmore sulfur to the hydrocarbon feed stream, if there is a deficiency ofsulfur, typically reduces coking and the amount of CO produced. Coke andCO are less desirable byproducts of the alkene steam cracking process.Eventually the amount of coke built up on the inside of the reactorreduces both flow rates and reaction efficiency such that the reactor isshut down and the coke is removed by a variety of processes.

[0014] The thermal decomposition temperature for SulfrZol 54 is about160° C. while the thermal decomposition temperature fordimethyldisulfide (DMDS) according to the same method is about 200° C.and that for dimethylsulfide (DMS) is about 250° C. so there is actuallya desirable decrease in the thermal decomposition temperature using thisparticular polysulfide. The vapor pressure of SulfrZol 54 is only 0.015psia while that of DMDS is 0.54 and that for DMS is 14.9 so there is adecrease in the worker exposure to airborne sulfur compounds. Furtherthe flash point of SulfrZol 54 is 100° C. while the flash point of DMDSis 15° C. and the flash point of DMS is <−18° C. Therefore there is asignificant decrease in the hazards of shipping and storing the sulfursource for the sulfiding reaction.

[0015] The term ethylene steam cracker is used herein to describe themore general alkene steam crackers. The primary alkenes generated fromthese types of crackers are well known and characterized. They includeethylene, propylene, isobutylene and possible some of the conjugateddienes such as butadiene or isoprene. The preferred embodiment for thisapplication is an ethylene steam cracker, which would produce primarilyethylene but possibly some other reaction products such as propylenewould be recovered. The gas feed for these types of reactors wouldcomprise hydrocarbon feed streams such as alkanes of substantially thesame number or possibly slightly higher number of carbon atoms as thedesired product. Desirably the hydrocarbon gas feed stream has a numberaverage carbon chain length of less than 10 and more desirably less than6. Thus it would be desirable in an ethylene steam cracker to have asubstantial amount of ethane in the feed stream. Similarly for a propanesteam cracker the feed would include an amount of propane. Thesehydrocarbon feed streams would typically be separated from petrochemicalsources such as natural gas, refinery streams, or byproduct streams froma manufacturing plant.

[0016] The hydrocarbon gas fed alkene steam crackers differsignificantly from liquid hydrocarbon fed alkene steam crackers. Aprimary difference is that the polysulfidic compounds will dissolve inliquid hydrocarbons facilitating their addition. In the gas fed steamcrackers the polysulfidic compounds need to be dispersed as droplets inthe gas stream. Gases vary from liquids in their viscosity, solventpower, and density. Liquids typically have densities of greater than0.6, 0.8 or 0.9 g/cc. Gases typically have densities below those such asless than 600 g/liter, less than 300 g/liter or less than 100 g/litereven under relatively high pressures.

[0017] The alkenes derived from steam cracking can be used to makepolyolefin polymers or can be further reacted to make other chemicalsincluding the various ethylenically unsaturated monomers. The chemistryof “ethylene” is better described in the Kirk-Othmer Encyclopedia ofChemical Technology, 3^(rd) edition volume 9 under the heading“ethylene”. Pages 400 through 431 of that book discusses various methodsof manufacturing ethylene including hydrolysis of hydrocarbons intubular reactors, design of pyrolysis heaters, typical once throughpyrolysis yields for various feedstocks (showing the recoverablehydrocarbons from cracking various feedstocks), table 5, pyrolysisheater characteristics (showing the operating conditions for a20,000-70,000 ton per year ethylene production heater unit), andrecovery and purification.

[0018] The ethylene steam crackers can be studied used quartz reactortubes partially enclosed in a furnace to achieve the desired operatingtemperatures. Articles exist in the published literature whereindividuals study a variety of reaction condition changes, including theincorporation of various sulfur compounds on process performance. Suchan article was title Simultaneous Thermal Cracking and Oxidation ofPropane to Propylene and Ethylene by Choudhary, V. R.; Rane, V. H.; andRajput, A. M. in AlChE Journal volume 44, no. 10 (1998-10) pp.2293-2301.

[0019] Generally in tubular steam crackers the reaction temperatures arefrom about 750 or 800 to about 900° C. and the residence times are fromabout 0.1 to about 0.6 seconds. The reactor walls can be at highertemperatures to achieve this reaction temperature. Low hydrocarbonpartial pressures are used. Steam is added to the feedstock to reducethe hydrocarbon partial pressure and the amount of carbon beingdeposited on the tube walls. The steam-to-hydrocarbon weight ratiosusually vary from 0.3 for ethane to as high as 1.0 for gas oil feeds.

[0020] While the invention has been explained in relation to itspreferred embodiments, it is to be understood that various modificationsthereof will become apparent to those skilled in the art upon readingthe specification. Therefore, it is to be understood that the inventiondisclosed herein is intended to cover such modifications that fallwithin the scope of the appended claims.

What is claimed is:
 1. In a process for converting a hydrocarbon gasstream to an alkene using a steam cracker including providing ahydrocarbon gas stream, heating the gas stream to at least 750° C. tothermally crack the hydrocarbon gas stream, and recovering the alkenereaction product wherein the improvement comprises adding at least 10ppm of an organic polysulfide in the form of dispersed droplets to saidsteam cracker based on the weight of said hydrocarbon gas stream.
 2. Ina process according to claim 1, where said organic polysulfide has aviscosity from about 1 to about 50 cps at 20° C. measured with aBrookfield viscometer.
 3. In a process according to claim 1, whereinsaid organic polysulfide has the formula R—S_(x)—R where R is a linearor branched alkyl of 3 to 15 carbon atoms and x is either an integerbetween 3 and 8 or R—S_(x)—R is a blend of compounds where x variesbetween 1 and
 8. 4. In a process according to claim 3, wherein R variesbetween 3 and 10 carbon atoms.
 5. In a process according to claim 3,wherein R is a t-butyl group.
 6. In a process according to claim 1,wherein said polysulfide is added using an atomizer with a gas assist.7. In a process according to claim 6, wherein the gas assist is an inertgas such as nitrogen or argon or a hydrocarbon gas stream having anumber average carbon chain length of less than
 4. 8. In a process forincreasing the sulfur content of a hydrocarbon gas stream including thesteps of measuring the flow of the hydrocarbon gas and adding a sulfurcontaining organic molecule, at a temperature of less than 100° C.,wherein the improvement comprises adding said sulfur containing organicmolecule as a fine dispersion of liquid polysulfide compound at atemperature below the thermal decomposition temperature of said liquidpolysulfide compound.
 9. A process according claim 8, wherein saidhydrocarbon gas stream has a number average carbon chain length of lessthan
 10. 10. A process according to claim to 9, wherein said hydrocarbongas stream has a number average carbon chain length of less than
 6. 11.A process according to claim 8, wherein said liquid polysulfide compoundhas the formula R—S_(x)—R where R is a linear or branched alkyl of 3 to15 carbon atoms and x is either an integer between 3 and 8 or R—S_(x)—Ris a blend of compounds where x varies between 1 and
 8. 12. A processaccording to claim 11, wherein R varies between 3 and
 10. 13. A processaccording to claim 12, wherein R comprises at least 50% t-butyl groups.14. A process according to claim 8, wherein said liquid polysulfidecompound is added as an aerosol dispersion using a gas assisted spraynozzle.
 15. A process according to claim 15, wherein said liquidpolysulfide compound is added in an amount of at least 10 or 50 ppmbased on the weight of said hydrocarbon gas.